Porcelain enameling of steel

ABSTRACT

PROCESS FOR THE PORCELAIN ENAMELING OF STEEL INVOLVING ELECTROLESSLY DEPOSITING NICKEL, COBALT AND COPPER ONTO CLEANED, PICKLED AND ETCHED STEEL PRIOR TO ENAMELING.

3,726,707 PORCELAIN ENAMELING OF STEEL Duane N. Prosser, Centerville, and Norman J. Sawdey,

Kettering, Ohio, assignors to General Motors Corporation, Detroit, Mich. No Drawing. Filed Oct. 4, 1971, Ser. No. 186,449 Int. Cl. C23c 3/02; C23d 5/00 US. Cl. 117-70 C 1 Claim ABSTRACT OF THE DISCLOSURE Process for the porcelain enameling of steel involving electrolessly depositing nickel, cobalt and copper onto cleaned, pickled and etched steel prior to enameling.

BACKGROUND OF THE INVENTION This invention relates to the art of porcelain enameling steel and more particularly to the treatment of the steel prior to the deposition of a porcelain enamel ground coat or cover coat.

Porcelainized steels are used in the manufacture of a variety of items including water cans, ovens, washing machines, etc. The basic process of porcelain enameling steel is well known and involves the principal steps of cleaning, pickling and etching the steel followed by coating with nickel, further acid rinsing and neutralization, and finally enameling and firing. The firing step fuses the enameling frit which is basically a glassy composition containing adherency promoting oxides. Typical examples of enameling frit compositions may be found in a number of texts known to those skilled in the art and especially, A. I. Andrews, Porcelain Enamels, second edition, Garrand Press, Champagne, Ill. (1961). The precise compositions of the frits are not part of the present invention, and hence will not be dealt with here in detail except to say that both ground and finish coats may be used with this invention. Application of the enameling frit to the surface may be by dipping or draining techniques such as are known to those skilled in the art. The basic process thus briefly outlined is described in more detail in a number of texts including Kirk-Othmer, Encyclopedia of Chemical Technology, vol. 8, second edition, Interscience Publishers, New York, NY. (1965). Such well known information will not be repeated here except to the extent necessary to a description of this improvement to the basic process.

Proper preparation of the steel is essential to a strong adherent uniform coating of frit. Good practice requires thorough cleaning, pickling and etching of the steel prior to coating with the frit. -A well recognized and important step in the preparation of the steel for reception of the frit is the step of nickel flashing in which a thin film of electroless nickel is deposited on the steel after etching. The nickel film retards corrosion of the steel and enhances the enamel bond. Nickel flashing is typically effected by immersing the steel for about 4-6 minutes in a solution containing about 1 oz. per gallon of a nickel salt, such as nickel sulfate hexahydrate or an equivalent, and a reducing agent such as sodium hypophosphite. The nickel coating weight on the finished piece ranges from about 0.04 g./ft. to about 0.12 g./ft. for good porcelain bonding.

From time to time nickel and its salts are in short supply and the cost of the available nickel becomes exorbitant resulting in increased costs to the enameler. In times such as this, attempts have been made to find less expensive, substitute nickel flashing baths having substantially lower nickel concentrations but which would produce the same results as are obtainable from the higher nickel concentrations. One such attempt involves United States Patent 0 merely substituting cobalt salts for some of the nickel salts. Acceptable adherency values are obtainable with these nickel-cobalt mixes, but only in a very narrow coating weight range, i.e. about 0.04 g./ft. to 0.06 g./ft. which is extremely difficult to control on a mass production basis.

It is an object of this invention to provide a porcelainizing process using a low nickel concentration flashing bath which has all the advantages of the higher nickel concentration flashing bath, as well as a broader coating weight range within which good porcelain bonding is possible. This and other objects of this invention will become more apparent from the detailed description which follows.

This invention comprehends a steel porcelainizing process including a steel preparation step which involves electrolessly depositing nickel, cobalt and copper onto the steel surface prior to applying the enameling frit. The Ni-Co-Cu coating is applied from an electroless deposition bath containing chemically reducible nickel, cobalt and copper salts and a reducing agent. The acceptable porcelainizing weight range for the Ni-Co-Cu deposit is about 0.035 g./ft. and 0.12 g./ft. More specifically, this invention comprehends the use of a preferred Ni-Co-Cu flashing bath comprising about 0.025 mole/ gal. (0.15 oz./ gal.) CoSO '7H O, about 0.028 mole/gal. (0.25 ozJgal.) Co(C H 0 -4H O, about 0.0088 mole/gal. (0.05 oz./ gal.) CuSO about 0.01 mole/gal. (0.1 oz./gal.) NiSO -6H O and about 0.011 mole/gal. (0.4 oz./gal.) NaH PO -H O. The bath is most effectively used at temperatures of about -170 F. and at a pH of about 4.5 to 5.0. The bath may be used effectively over a broad cation concentration range in which the cobalt ranges from about 0.03 mole/gal. to about 0.21 mole/gal, the copper from about 0.0011 mole/gal. to about 0.088 mole/ gaL, and the nickel from about 0.005 mole/gal. to about 0.032 mole/gal. The concentration of the reducing agent (i.e. sodium hypophosphite) will vary in proportion to the concentration of the metallic salts requiring reduction. This will usually be about 0.0026 mole/ gal. to about 0.013 mole/ gal. A number of salts may be used for the several cations involved here. Hence, for the copper ion, copper acetate, copper sulfate or copper carbonate may be used. Other water soluble, readily reducible copper salts known to those in the plating art may also be used. For the nickel ion, nickel sulfate, nickel fluoride, nickel car bonate, nickel acetate, as Well as other readily reducible nickel salts may be used. For the cobalt ion, cobalt sulfate and cobalt acetate are useful.

The following is a specific example of the process which is the subject of this invention. A sheet of porcelain enameling steel is thoroughly cleaned by passing it through a number of cleaning and rinsing cycles followed by a 0.6 minute, F. sulfuric acid (4.0%-6.0%) pickle; a 1.5 minute, 180+ F. ferric sulfate (l.8%-2.5%) etch; a 1 minute, 155 F. sulfuric acid (l.0%2.0%) rinse; a 2.4 minute, 170 F. sulfuric acid (4.06.0%) pickle; and a 0.6 minute, dilute sulfuric acid (0.2%0.3%) rinse prior to the cobalt-nickel-copper flash of this invention. The thusly treated steel is then sprayed with a F. Co-Ni-Cu flashing solution comprising 0.25 oz./gal. of cobalt sulfate, 0.25 oz./ gal. of cobalt acetate, 0.05 oz./gal. of copper sulfate, 0.1 oz./gal. nickel sulfate and 0.40 oz./gal. sodium hypophosphite. Electroless deposition continues for a time suflicient to deposit about 0.05 g./ft. of the Co-Ni-Cu coating. The thusly coated sample is then rinsed in dilute sulfuric acid (02-03% neutralized and finally rinsed in deionized water and dried. An appropriate slip of porcelain frit is then applied to the surface aud fired. During the aforesaid treatment it will be noted that a copper smut forms on the plated surface but a substantial portion of it is removed during the subsequent rinsing and neutralizing stages.

While we have described our invention in the terms set forth above, we do not intend to be limited to any extent other than hereinafter set forth.

We claim:

1. In the process of porcelain enameling steel comprising the principal steps of cleaning it, pickling it, etching it, coating it with electrolessly deposited metal, acid rinsing it, neutralizing it, drying it, coating it with frit and firing it, the improvement to said metal coating step comprising contacting said steel with a 140 F.-170 F. aqueous solution comprising Mole/gal. CO' 0.03-0.21 Cu 0.001l0.088 Ni++ 0.00-0032 (H PO 00026-0013 for a time sufiicient to deposit at least 0.035 g./ft. and less than 0.12 gJft. of a Ni-Co-Cu layer on said steel to promote an adherent bond between said steel and said fn't.

References Cited UNITED STATES PATENTS Chester 117-70 D X Sutphen et a1.

Chester 117-70 D X Eisenberg 117-130 E Porter et a1 117-71 M X Schunemann 117-71 M Ziehr et a1. 117-70 C X Saubestre et al. 106-1 Etter 117-130 E X Clauss et a1. 106-1 X ALFRED L. LEAVITT, Primary Examiner I. R. BATTEN, JR., Assistant Examiner US. Cl. X.R.

29-195; 106-1; 117 A, 71 M, E 

